The present invention relates to a merchandiser type refrigerated display case or cabinet used primarily in retail food and supermarket outlets. More specifically, it relates to a combination refrigerated gas and ambient air defrost means to be employed during the defrosting of such cases in an energy efficient manner.
The term "refrigerated" in accordance with the present invention is intended to incorporate those cases maintained at a temperature at or in excess of 32.degree. F. such as display cases utilized for display of milk and fresh foods, and those cases maintained below 32.degree. F., such as frozen food cases.
In the operation of all types of refrigerated display cabinets, it is desirable to include a system for automatically defrosting the refrigeration coils. The defrost cycle can be actuated either at set periodic time intervals or when the defrost build-up within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation, it is possible to avoid any significant frost build-up within the display case such that inoperability and spoilage of the stored food products would occur.
There have been three different approaches for defrosting refrigerated display cabinets in this art. These are: utilizing electric resistance heaters; passing a compressed refrigerant gas having a high specific heat through a refrigeration coil; and circulating ambient air through a conduit in which refrigeration coils are positioned. A problem which occurs when using electric resistance heaters or compressed refrigerant gas as the defrosting means is that the heat released in the display cabinets or cases by such means during the defrost cycle can cause food spoilage and/or lesser undesirable changes if not carefully handled. This problem can be largely overcome by defrosting with an incoming ambient air band which is ejected from the case being defrosted in a manner to avoid contacting the stored food products with the ambient air. By use of the ambient air as an external heat source, a more effective defrost can be possible.
Another problem with respect to the use of electric resistance heaters is that these require substantial electric power consumption and are therefore relatively expensive to operate.
The use of ambient air as the defrost medium has a number of advantages including lower operating costs. The ambient air which is substantially warmer than the refrigerated air, is passed over the cooling coils to warm them sufficiently to defrost them. Air defrost utilizes heat extracted from the ambient air in two forms: sensible heat is extracted by dropping the air temperature and latent heat is released in the cooling coil area through the condensation of water vapor in the air. As a rule of thumb, moisture releases about 1300 BTU's per pound when condensed. It has been found that under certain store conditions of temperature and humidity, the air defrost method alone may not be completely adequate to defrost the cooling coils of certain types of frozen food cases rapidly enough to prevent partial defrosting and spoilage of food products stored in the case.
In some environmental conditions, the ambient humidity in the store will be relatively low. Under such low humidity conditions, not enough latent heat can be extracted from the air to defrost the cooling coils in a sufficiently short period of time to prevent the ice cream from melting. It has been found that, at such times, air defrost techniques alone are not completely effective to defrost ice cream containing cases without suffering a certain amount of product spoilage due to the defrosting and refreezing of the ice cream. These types of problems are present for other refrigerated display cases to an extent that additional heat input during the defrost cycle is sometimes considered advantageous.
While the utilization of an ambient air band is the most energy efficient of the three above-described techniques, it is also desirable and economically and commercially necessary to defrost refrigerated display cases quickly to prevent undesirable defrosting of the products stored therein. Applicant's U.S. Pat. No. 4,265,092 discloses and claims a supplemental electric heater mechanism for providing additional heat to the ambient air stream used for defrosting. Another possibility is to utilize a compressed refrigerant gas defrost means to enable heat to be released directly in the refrigeration coil area through the metal walls of the coils whereby a quick defrosting of the frost and ice coating on the coils can be achieved. The combination of a gas defrost means with an ambient air band can result in a highly effective and efficient defrost system. The refrigerant gas produces high heat transfer internal to the coils and the ambient air defrosts other air conduits and drain areas in the cases by use of the lower temperature ambient air. Such a defrost system is usable in a wide range of refrigerated display cases.
Most commercially used refrigerated cases having umimpeded access openings therein are of two types:
1. Open top case having a single air conduit;
2. Open front cases having single or multiple air conduits surrounding the product display space.
When hot gas defrost is used in such refrigerated cases together with an ambient air band defrost, the avoidance of contacting stored products with ambient air becomes even more important because of the elevated temperature of the circulated air band. The prior art has not recognized the importance of this problem and/or has not evolved operative solutions.
One approach to the use of gas defrost means is set forth in U.S. Pat. No. 4,285,204 to Vana. This patent notes that recirculated air can be used to distribute the heat provided by gas defrost within the air conduit. Thus this patent discloses the recirculation of the same air bands in a dual compartment open top case so that the defrost air band is the same as the circulated air band, and hence does not provide for the continuous intake of ambient air. It is stated that the circulation direction of the air bands should be reversed during part of the defrost cycle in order to help defrost certain problem areas which are stated to be the drain areas and the central flue which are heavily frosted during the refrigeration cycle. The flow pattern of air established is such that comingling of ambient air and the recirculated air band over the product display space unavoidably occurs whereby ambient air is moved into contact with the stored products. Further, no unidirectional ambient air band flow through the refrigeration coil means during the entire defrost cycle is established. Also this patent states that the recirculated air path does extend close to the chilled products and loses heat thereto when the case is in a defrost mode.
U.S. Pat. No. 4,320,631 also to Vana describes an auxiliary defrost air discharge mechanism, but does not utilize hot gas defrost, whereby it is limited solely to air defrost.
U.S. Pat. No. 3,383,877 to Liebermann et al discloses the use of hot gas defrost in a refrigerated case which is also provided with air circulation fans; however, these fans are deenergized during the hot gas defrost operation whereby an ambient air band is not generated for providing additional heat input for defrosting. Since no air is circulated during the defrost cycle, there is a reduced occurrence of ambient air coming into contact with stored products, but no combination defrosting takes place.
U.S. Pat. No. 4,026,121 to Aokage et al describes a refrigerated case in which defrosting is accomplished by use of an ambient air band without hot gas defrost energy input. In this patent, the protective air band which prevents moist ambient air from entering the product display space during a refrigeration cycle is terminated during a defrost cycle so that ambient air can come into contact with products stored in this open front case.
U.S Pat. No. 4,120,174 to Johnston discloses an open top case in which a reverse direction ambient air band is utilized for defrosting. There is no disclosure of hot gas defrost to be used in conjunction with this defrost air band. The ambient air band which is ejected from the air conduit in a nearly vertical direction so that the defrost air which often tends to be colder than the surrounding air can fall backward into the open top of the case and thereby entrain ambient air which will come into contact with the stored products.
In distinction to the above-described prior art, the applicant has discovered that a combination of hot gas defrost and an ambient air defrost band can be efficiently employed during defrost cycles in both open top and open front cases providing that means are provided to enable the ambient air band to flow into the air conduit of the refrigerated cabinet and for the defrost air band to exit from the case in a manner so as to prevent ambient air from being forced into the product display space through the access opening. The combination hot gas and the ambient air defrost permits quick defrosting of all parts of the refrigerated case while maintaining refrigerated product quality.